Many circuits exist for producing a fixed output signal whenever an alternating input signal exceeds a given value. In some systems, such as in a frequency modulated continuous wave radar (FM-CW), a complex alternating signal may undesirably be produced by the radar in which on a particular cycle the signal level reaches a first low value but either never reaches a higher desired value or reaches it at such a later time as to indicate that the signal is not of an acceptable shape to be processed in the processing portion of the system. Conventional threshold circuits do not handle and detect such a complex waveform as described.